In the formation of integrated circuits (IC), thin films containing metal and metalloid elements are often deposited upon the surface of a semiconductor substrate. Thin films provide conductive and ohmic contacts in the circuits and between the various devices of an IC. For example, a thin film of a desired metal might be applied to the exposed surface of a contact or via hole on a semiconductor substrate, with the film passing through the inculpative layers on the substrate to provide plugs of conductive material for the purpose of making interconnections across the insulating layers.
Titanium nitride (TiN) films are commonly used in semiconductor stacks as barrier layers between the substrate (silicon, silicon dioxide, or any insulator) and metal (aluminum wires, tungsten plugs or aluminum plugs). Similarly, titanium (Ti) films are used in semiconductor stacks as wetting layers between a silicon (Si) substrate and the barrier layer (TiN or tungsten (W)) and also to form Ti-silicide contacts for submicron drains and gates. Magnetron sputtering of titanium targets is widely used to deposit thin films of Ti and TiN onto flat and patterned substrates in the manufacture of integrated circuits.
The primary requirement for Ti or TiN films deposited on flat substrates (i.e. blanket deposition) is the thickness uniformity of the film across the eight inch diameter Si wafer substrate. Film uniformity is improved by maintaining a grain size less than 50 .mu.m, by maintaining an optimal crystallographic orientation in the target, and by maintaining these two parameters consistent across the target diameter and thickness. On the other hand, the requirement for Ti or TiN films deposited on patterned substrates (into vias) is the percentage of the area covered by the film across the bottom surface of a via whose width may range from 0.25 .mu.m to 2.0 .mu.m. Bottom coverage is improved by eliminating sputtered atoms whose trajectories are not perpendicular to the bottom surface of the via, either by placing a collimator between the target and the substrate or by increasing the distance between the target and substrate.
Two problems arise in conventional Ti targets for blanket deposition--large grains and strong crystallographic orientation. A target with undesirably large grains or strong crystallographic orientation has undesirable effects on the sputter deposition rate and film uniformity. The large grains, ranging in size from 10 mm to 50 mm, form as a result of the high temperatures involved in the bulk manufacture of high purity Ti by electron-beam melting and casting of billets ranging from 12 inches to 30 inches in diameter. Grain refiners cannot be used since they contaminate the target and hence contaminate the deposited film, contaminate its electrical properties, and ultimately contaminate the semiconductor device itself. Consequently, conventional Ti sputtering targets have been fabricated by hot working, cold working, and heat treatment to have grain size less than 50 .mu.m. The strong crystallographic orientation forms as a result of the choice of temperatures used in the above hot-working, cold-working and heat treatment. Hence, the fabrication process for conventional Ti targets is designed to yield a weak crystallographic orientation perpendicular to the sputtering direction, such as that listed below using the Miller Index for crystallographic orientation:
______________________________________ (100) (002) (101) (102) (110) (103) (112) ______________________________________ &lt;5% 10-25% &lt;10% 15-25% &lt;10% 40-60% &lt;10% ______________________________________
Thus, on the target sputter face, less than about 5% of the grains have the (100) crystallographic orientation, about 10% to about 25% of the grains have the (002) crystallographic orientation, less than about 10% of the grains have the (101) and (110) crystallographic orientation, about 15% to about 25% of the grains have the (102) crystallographic orientation, about 40% to about 60% of the grains have the (103) crystallographic orientation, and less than about 10% of the grains have the (112) crystallographic orientation. Such a target has a cosine angular distribution of sputtered atoms as a result of the summation of the anisotropic emissions from grains having different crystallographic orientations. It is known that the film uniformity of such a uniform fine grain and randomly oriented target is superior to a target with a strong crystallographic orientation.
Two problems arise when using conventional Ti target for via deposition by collimation or long-throw sputtering--reduced deposition rate and lower yields for submicron vias. Conventional Ti targets have cosine angular distribution so that only 10% of the sputtered atoms are emitted perpendicular to the via bottom surface and the remainder are intercepted by the collimator, thus reducing the sputtering deposition rate. It is known that a lower sputtering deposition rate produces a Ti film with a weak (002) orientation, which is not preferred since it deleteriously affects the crystallographic orientation of the subsequent aluminum layer. Therefore, a target with a strongly overcosine angular distribution of sputtered atoms would offer a higher percentage of sputtered neutrals perpendicular to the via bottom surface. This would result in improved film quality, i.e. strong (002) orientation, and improved target utilization, which would reduce cost of ownership. However, this must be coupled with uniform and fine grains in order to achieve the increase in deposition rate while maintaining uniformity of bottom coverage across the eight inch diameter Si wafer substrate.
It is known that the sputter deposition rate through a collimator is maximized if the target has a preferred (103) crystallographic orientation and the (002) orientation is controlled below a limiting value. Therefore, a titanium target with a near-ideal (103) crystallographic orientation provides a higher deposition rate than one with a low (103) or a high (002) or a random crystal orientation. This occurs because sputtering targets with greater than 80% of (103) crystallographic orientation have a strongly overcosine distribution. This in turn means that a higher percentage of sputtered atoms are emitted perpendicular to the substrate surface resulting in an increased transmission rate through a collimator. An additional benefit is improved efficiency of bottom coverage for narrower and deeper vias, such as vias that are 0.5 .mu.m wide by 1 .mu.m deep. Step coverage, defined as the ratio of film thickness at the bottom versus at the overhang, and bottom coverage, defined as the percentage of area covered by film, are improved by 100% to 500% when a strongly overcosine angular distribution of sputtered atoms is used.
Several other combinations have been tried to form an improved target or improved process for bottom coverage. For example, a polycrystalline sputtering target with a preferred (002) crystallographic orientation has been used. A single crystal target with any preferred orientation has also been used. Finally, ionization of the sputtered neutral atoms and orienting the depositing neutrals by the means of a magnetic field on the plasma between the target and the substrate has been used. These combinations are not satisfactory because of the tradeoff in sputter deposition rate and uniformity of bottom coverage across the substrate diameter. For example, when using a sputtering target with a preferred (002) crystallographic orientation, the Ti films deposited by collimation sputtering have a very low (002) crystallographic orientation, which is not preferred. Alternatively, using a single crystal sputtering target results in non-uniform deposition across the substrate diameter.
There is a need, then, to control a titanium sputter target's grain size, crystallographic orientation and composition in order to provide a higher sputter deposition rate while maintaining uniformity across the substrate diameter to yield increased efficiency of collimation sputtering and of bottom coverage of submicron vias.